1. Field of the Invention
The present invention relates to a measurement method, an exposure method, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate fine semiconductor devices such as a semiconductor memory and logic circuit using photolithography. The projection exposure apparatus projects and transfers a circuit pattern drawn on a reticle (mask) onto, for example, a wafer via a projection optical system. Along with the recent demand for micropatterning semiconductor devices, the projection exposure apparatus is desired to transfer a pattern having a size half the exposure wavelength or less by exposure, and to further improve the resolution (i.e., attain a higher resolution).
The exposure apparatus generally achieves a higher resolution by shortening the exposure wavelength and increasing the numerical aperture (NA) of the projection optical system (attaining a high-NA projection optical system). However, only shortening the exposure wavelength and attaining a high-NA projection optical system are insufficient to achieve a high-resolution exposure apparatus. It is also necessary to improve the performance of the exposure apparatus itself. For example, some recent projection optical systems have achieved remarkable improvements in regard to aberrations. Also, some recent illumination optical systems have achieved polarized illumination in place of the conventional non-polarized illumination. The polarized illumination here means an illumination method of controlling the polarization state of illumination light in accordance with the pattern of a reticle.
To maintain high performance, the exposure apparatus comprises a measurement unit for measuring various optical characteristics, and a correction unit for correcting the optical characteristics on the basis of the measurement result obtained by the measurement unit. For example, there is proposed an exposure apparatus which can measure and correct the aberration of a projection optical system. Examples of measurable optical characteristics are the NA of the projection optical system, the polarization state of illumination light, the curvature of field, the exposure dose, the exposure dose uniformity, the illumination light distribution, and the Jones matrix of the projection optical system, in addition to the aberration of the projection optical system.
Exposure apparatuses are roughly classified into two types that is, an exposure apparatus (stepper) of a step-and-repeat scheme and an exposure apparatus (scanner) of a step-and-scan scheme. The scanner is an exposure apparatus which supports the recent increase in resolution because it is more amenable to increases in the NA than the stepper.
The scanner scans a reticle and wafer to transfer the pattern of the reticle onto the wafer. If the movement of the reticle (reticle stage) is not correctly synchronized with that of the wafer (wafer stage) during scanning, the imaging position shifts from a desired one, resulting in deterioration in an aerial image due to image blurring; that is, distortion (scan distortion) occurs during scanning. The scan distortion is a factor of deterioration in the aerial image, which never occurs in the stepper. Because the scan distortion has little influence on the aerial image, it has not been problematic up to now. However, along with recent increases in the performance of exposure apparatuses, the scan distortion is becoming nonnegligible.
To cope with this situation, attempts have been made to measure and correct the scan distortion. For example, the distortion can be measured by exposing a resist to measure the positional shift of the light intensity distribution. U.S. Pat. No. 6,803,554 proposes a technique of forming minute pinholes in a light intensity sensor such as a CCD (Charge Coupled Device) in a grid pattern to measure the light intensity distribution on a wafer surface. Japanese Patent Laid-Open No. 2002-14005 proposes a technique of inserting a minute slit and light intensity sensor on the image plane of a projection optical system to measure the light intensity distribution on a wafer surface.
Unfortunately, measurement of the distortion (scan distortion) that occurs during scanning using prior art presents great difficulty. For example, the distortion measured by exposing the resist is the accumulated scan distortion. It is therefore impossible to measure the degree of distortion which has occurred at a certain time during scanning. U.S. Pat. No. 6,803,554 and Japanese Patent Laid-Open No. 2002-14005 disclose no method of measuring the shift of the imaging position during scanning. In other words, U.S. Pat. No. 6,803,554 and Japanese Patent Laid-Open No. 2002-14005 merely measure the accumulated light intensity distribution after completing scanning exposure, or a light intensity distribution after stationary exposure.